The Action of aldose reductase (EC 1.1.1.21; "low-Km" aldehyde reductase; ALR2) is implicated in the etiology of diabetic complications of the eye, kidney, heart, and nerve. The ability of specific aldose reductase inhibitors (ARIs) to prevent the development of complications (basement membrane thickening, renal hypertrophy, defective axonal transport and nerve conduction velocity) in various tissues has confirmed the central role of ALR2 in normal and diabetic metabolism. This conclusion is supported by immunologic evidence for phylogenetic conservation of the mammalian ALR2 gene product. Homogeneous bovine kidney ALR2 (BKALR2) is virtually identical to bovine lens ALR2, but only after the BKALR2 has undergone a slow, thiol-dependent activation to a stable form. The activation process, which results in alteration of the kinetic behavior (decreased sensitivity to inhibition by Sorbinil, decreased stimulation by sulfate, appearance of non-linearity in the double-reciprocal plot for glyceraldehyde), is relevant to the activation and variable ARI sensitivity seen in human tissues. Further mechanistic studies using homogeneous BKALR2 will focus on characterization of the activation process using kinetic and immunologic techniques, identification of the physiological substrate by structure-activity studies, and determination of the mode of ARI inhibition. The extension of these studies to human kidney ALR2 and "high-Km" aldehyde reductase will utilize mon-specific rabbit anti-BKALR2 for the isolation and characterization of HKALR2. Key experiments, based on the results from the bovine system, will evaluate the mode and sensitivity to ARI inhibition, and the role of activation (as documented for BKALR2) in the human system.